Would it have been immoral to disassemble Frankenstein's monster if the donors of those body parts could have been restored into living people? It seems like the same scenario to me.

While it's not completely the same scenario it does come close. The true immorality was Frankenstein stealing the body parts in the first place whether or not the bodies can be reanimated. At least in the eyes "of opt-in" donor countries. And to further complicate the situation the Doctor that declares death cannot perform the transplant either, which puts our doctor in a double bind. He is wrong which ever way he turns.

In fact he lost his chance to get out of the situation when he researched a solution to the problem of returning Tuvok and Neelix. That suggests that no one on Voyager was interested in declaring either one of the two men dead.

The last can of worms is Vulcan mysticism, I believe (and correct me if I am wrong here) that Tuvok did not subscribe to the concept of the Katra. But that does not preclude his wife or children's' belief, or given Vulcan lifespans he may even have parents still alive that could have an opinion on the matter. Remember even Opt-out donor systems can be denied the deceased if someone acting as a guardian says no.

Janeway is right when she says someone has to speak for the two members that cannot speak for themselves. She may even be bound by federation law to make the decision she did.

Like I said earlier, this looks like a cold way of approaching the situation, but this is why justice is depicted as wearing a blindfold. There is way too much emotion to go any other route. With no good answer, one can only choose the lawful one.

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"My name is Matai Shang and I do not exist, indeed I work very hard at it."